Surface treated steel plate

ABSTRACT

THIS INVENTION RELATES TO THE SURFACE TREATMENT OF STEEL PLATES OF SHEETS AND DISCLOSES A NOVEL STEEL SHEET STRUCTURE, WHEREIN THE STEEL SHEET IS FIRST ELECTROLYTICALLY SURFACE COATED WITH A METALLIC CHROMIUM LAYER OF A THICKNESS NOT EXCEEDING ABOUT 0.1 MICRON AND IS THEN ELECTROLYTICALLY PLATED WITH A CHROMATE FILM OF A THICKNESS NOT EXCEEDING 0.1 MG./DM.2. THE CHROMIUM LAYER THICKNESS SHOULD PREFERABLY NOT BE LESS THAN 0.0016 MICRON. THE INVENTIVE STEEL SHEET CONSTITUTES A SUPERIOR CAN STOCK AND MAY BE PROVIDED WITH A TOP COAT OF ORGANIC COMPOSITION.

SURFACE TREATED STEEL PLATE Original Filed March 23; 1967 FIG] INVENTORSfilRam 00H! uoumn BY aspnu Yn/vnau WWW United States Patent 3,827,866SURFACE TREATED STEEL PLATE Hiromu Uchida and Osamu Yanabu, Himeji,Japan, assignors to Nippon Steel Corporation Continuation of abandonedapplication Ser. No. 625,421, Mar. 23, 1967. This application Jan. 26,1971, Ser.

Int. Cl. B32b /00 US. Cl. 29-195 10 Claims ABSTRACT OF THE DISCLOSUREThis application is a continuation of application 625,- 421, filed Mar.23, 1967, now abandoned.

The invention also discloses a two-step process for producing the sheet.Pursuant to this process the metallic chromium layer is firstelectrolytically deposited on the steel sheet surface, whereupon thechromate film is electrolytically deposited on the metallic chromiumlayer. An important feature of the invention is that the chromatetreatment is carried out in a solution which is devoid of a sulphuricacid radical having a tendency to deposit metallic chromium. Particularprocess conditions in re spect to bath composition, current density andtreatment time are disclosed.

SUMMARY OF THE INVENTION This invention generally relates to the surfacecoating of steel and is particularly directed to novel surfacetreatedsteel plates or sheets in which a metallic chromium layer is directlyplated on the steel sheet surface, a chromate layer thereupon beingelectrolytically superimposed on the metallic chromium layer. Theinvention is also concerned with a procedure for obtaining the novelsurface coated steel sheets and encompasses the subsequent coating ofthe surface-treated steel sheets with organic coating compositions.

Relatively thin-gauge steel sheets imparted with the inventivechromium-chromate layers have proved to be superior can stock.

Various proposals have been put forward in the art for the purpose ofchromate-treating steel plates, to wit, directly coating the steel sheetsurface with a chromate film. According to one prior art procedure, thesteel plate to be chromate coated is subjected to cathodic treatment inan aqueous solution containing hexavalent chromium ions and trivalentchromium ions.

According to another suggestion, the cathodic treat ment of the steelplate is effected in an aqueous solution containing chromic acidanhydride, trivalent chromium ions, phosphoric acid and boric acid. Ithas also been proposed that the cathodic treatment of the steel platecan be effected in a liquid system prepared by adding chromic acid ionsto a phosphoric acid solution, the solution also comprising acidicphosphates of zinc, magnesium, calcium and manganese. In a more recentmethod, the steel plate is subjected to treatment in an aqueous solutionof chromic acid, the solution also containing certain reducing materialswhich have a tendency to reduce the chromic acid to form a film ofreduced chromic acid on 3,827,866 Patented Aug. 6, 1974 the steel plate.The thus treated steel plate is then heated to about 250 F. It has alsorecently been suggested to effect the cathodic treatment of the steelplate in a liquid containing phosphoric acid and dichromate.

US. Pat. 2,769,744 teaches still another procedure, according to whichthe cathodic treatment is performed in a bath prepared by addingphosphoric acid to a chromic acid anhydride solution. Similar processeshave been taught in US. Pats. 2,733,199 and 2,780,592, according towhich the cathodic treatment is performed in an aqueous bath liquidcontaining boric acid or its salts and chromic acid anhydride in aqueoussolution.

Finally, U.S. Pats. 2,768,103, 2,768,104 and 2,777,785 suggest aprocedure in which the steel plate is immersed in a liquid prepared byadding a reducing agent such as, for example, cane sugar, to an aqueoussolution of chromic acid anhydride, whereupon the plate is dried underthe formation of a chromate film. It has also been suggested that thecathodic treatment be performed in a chromic acid containing bathenriched by certain compounds that have a tendency to accelerate thefilm formation such as, for example, catechol-disulfonic acid, phenoldisulfonic acid and the like compounds. Other film inducing agents havebeen suggested for this purpose as, for example, selenic acid, zirconicacid, potassium fluoride, sodium-silico fluoride, titanium potassiumfluoride, perchloric acid and ammonium molybdate.

It should be appreciated that in all the prior art procedures referredto, the chromate film or layer is directly formed on the steel surface.This layer or film of the prior art procedures is mainly composed ofchromium oxide and chromium hydrates, in which trivalent chromium is theprevaling constituent together with hexavalent chromium salts. Althoughthe chemical structure of these surface layers has not been exactlyidentified, due to the amorphism of the structure, it is generallyaccepted by those skilled in this art that trivalent chromium oxide andits hydrates forms a film which adheres to the steel plate surface in anirregular gel state or formation, this film being covered by hexavalentchromium salts. In this manner, the steel surface is imparted withcorrosion resistance due to the passivation of the structure.

A variety of modifications have lately been proposed for the purpose ofimproving the surface coating characteristics. According to thesemodifications, the proportions of the trivalent and hexavalent chromiumare changed with or without the presence of a phosphate film or layer.In any event, the main components of the surface films formed on thesteel plate surface are of theindicated nature and are generallyreferred to in the art as chromate films. In this connection it shouldbe appreciated that the term chromate as used in the field of metalsurface treatment has a particular meaning and generally refers totrivalent chromium oxide and its hydrates, the trivalent chromium oxidesometimes being admixed with hexavalent chromium oxide and its hydrates.I

As previously set forth, the prior art procedures result in theformation of a chromate film directly on the surface of the steel plate,and no metallic chromium layer is interposed between the steel surfaceand the chromate film. This is due to the composition of the bath ortreating liquid and the prevailing cathodic treatment condi- Accordingto the second group, the steel plate is subjected to cathodic treatmentin which the electro-plating is performed in a liquid of similarcomposition with the steel plate constituting the cathode.

'It is well known by those skilled in the art that, although the directformation of a chromate film on the steel plate surface improves thecorrosion resistance of the steel, the chromate film is not fullysatisfactory or adequate for a variety of purposes to which the steelplate may be put. Thus, for example, the corrosion resistance ofchromate treated steel plates is generally satisfactory as long as theplate remains uncoated with organic coatings, but once an organiccoating has been applied onto the chromate layer and the plates havebeen formed, the chemical resistance of the resulting product isunsatisfactory as evidenced by chemical resistance tests. In thisconnection it should be appreciated that if the chromate treated steelplates are to be used, for example, for the manufacture of cans, anadditional top coating with organic compositions is necessary and indeedrequired by the laws of most countries. The reason that thecharacteristics of chromate treated steel plates to which an organiccoating has been applied are unsatisfactory, is due to the fact that,although relatively good adherence is obtained between the organiccoating composition and the chromate film, the adherence between thesteel surface and the chromate film is unsatisfactory. It has thus beenfound that when a chromate treated steel plate, subsequently coated withan organic lacquer or varnish composition, is immersed in a solutioncontaining citric acid and sulphuric acid, the acidic solutionpenetrates through the organic coating film to corrode the chromate filmand the steel surface progressively, until the organic film peels off.

US. Pat. 3,113,845 is concerned with surface coated steel sheets for canstock purposes, wherein the steel sheet is electrolytically coated witha metallic chromium layer of predetermined thickness, the chromium layerin turn being coated with an organic coating composition. This patentconstitutes an important improvement in the art of surface treatingsteel plates and teaches the production of steel plates which aresuperior to those coated with a chromate film. The US. patent referredto contains specific disclosures concerning the thickness of themetallic chromium layer, the maximum thickness of the layer beingindicated as 0.1 micron. This maximum thickness of the chromium layer isimportant from the viewpoint of quality. In respect to the lower limitof the chromium layer thickness, consideration has been given to theminimum thickness necessary to obtain sufficient corrosion resistance,which latter should be equal if not superior to that of ordinary tinplates used as can stock. Chromium plated can stock of the naturedisclosed in US. Pat. 3,113,845 and having a chromium layer thickness offrom 0005-01 micron has been favorably received by the trade and theindustry and is now widely accepted as superior and low-priced can stockmaterial. It has also found other commercial applications.

It is a primary object of the present invention still further to improveon the qualities and characteristics of steel plates which have beendirectly chromium plated such as disclosed in US. Pat. 3,113,845.

According to the US. patent referred to, the steel plate, after themetallic chromium layer has been electrolytically deposited thereon, maybe chemically treated with a diluted chromic acid solution. Thistreatment, according to the prior patent, is effected in a tank intowhich the chromium plated steel plate is dipped. Extensive experimentshave indicated that this dipping treatment does not yield significantlyimproved results and, in spite of the dipping into chromic acidsolution, the final plate is susceptible to pit corrosion before theorganic coating has been applied, when the plate is subjected to saltspray testing. Further, it has been ascertained that the plates do notexhibit improved corrosion resistance even after the organic coating isapplied subsequent to the chromic acid dipping treatment. It has thusbeen established that if the final products manufactured from the platesand after they have been topcoated with the organic composition, aresubjected to corrosion tests, no improved results are obtained. This isparticularly so if the chromium layer thickness is less than 0.01micron. With layer thicknesses in the range of 0.01-0.1 micron betterresults could be observed. Investigations have revealed that when achromium plated steel plate is dipped into a chromic acid solution, astaught in the prior patent, a chromate film is merely formed at the pinhole areas of the steel plate, but no chromate film formation can beobserved on the metallic chromium layer. In other words, only at theexposed areas where no chromium layer is formed on the steel surface,does a chromate film adhere to the steel proper, thus covering the pinholes.

Our studies were then directed towards finding etficient treatmentconditions for forming a chromate film on the metallic chromium layer bymeans of cathodic treatment and as a result of such studies, anefficient and simple method was developed electrolytically to deposit achr0- mate film on the metallic chromium layer previously electroplatedonto the steel surface. In accordance with this procedure which thusforms a primary aspect of the present invention, the pin hole portionsare also covered by a chromate film (chromium oxide and/or chromiumhydrate film) so that the chromate layer uniformly covers the entire topsurface of the steel plate and overlies both the metallic chromium filmon the sheet and the pin hole areas. It was further established thatsubsequent coating of the chromate layer with organic compositionsresults in a superior product and that such product has excellentcorrosion resistance and adherence characteristics of the variouslayers. The chromate film formed in accordance with this invention has,moreover, desirable density characteristics.

Steel sheets first imparted electrolytically with a metallic chromiumlayer and then with an electrolytically deposited chromate layer exhibitexcellent results in salt spray tests before the sheets are top-coatedwith organic compositions. Further, the products show improved corrosionresistance, both with and without organic top coats, if the chromiumlayer thickness is between 0.01-0.1 micron. Moreover, the corrosionresistance of the inventive product is at least equal, if not better,than that of tin plates even if the thickness of the metallic chromiumlayer is as thin as 0.001 micron. From a practical point of view it hasbeen established that the preferred thickness range for the metallicchromium layer is 0.00l6-0.l micron.

Accordingly, the present invention provides for chromium plated steelsheets which have electrolytically superimposed thereon chromate filmswhich in turn may be covered with organic coatings. The inventive sheetshave superior surface properties and are economically produced in asimple manner. Since the thickness of the various layers is extremelythin and the price of chromium is less than that of tin, the inventivesheets can be more economically produced than tin plates.

Briefly, therefore, the invention essentially resides in the provisionof a surface treated steel plate having directly surface plated thereona metallic chromium layer of a thickness of between about 0.00160.1micron, the chro mium layer, in turn, being electrolytically coated witha chromate film which overlies both the chromium layer and the pin holeportions.

In accordance with this invention it has been ascertained that thethickness of the chromate film superimposed onto the metallic chromiumlayer should not exceed 0.1 mg./ dm. The chromium-chromate plated steelsheet may thereafter be top coated with an organic composition as isknown in the art and as disclosed, for example, in US. Pat. 3,113,845,previously referred to.

As stated, the thickness of the chromate film electrolytieallysuperimposed onto the chromium layer should not exceed 0.1 mg./dm. Thereason for limiting the thickness of the chromate layer to the indicatedupper value is primarily due to the fact that with thicker chromatefilms, the surface color tone of I products made from the surfacetreated sheets does not attain any significant metallic lustre or gloss.If the chromate layer is thicker, the surface color or hue has atendency to be of a grayish or bluish tint, thus lowering the commercialvalue of the product. Furthermore, thicker chromate films may result inthe ultimate peelingoff of the organic coating which is normally appliedas top coat. This has been established in undercut film corrosion tests.Generally, the adherence characteristics suffer if the chromate layerexceeds the indicated limit and, moreover, discoloration may take placeduring heating of the product in an air atmosphere. This is demonstratedby the results of tests as tabulated in the following Table 1.

TABLE 1 Discolora- Thickness of Thickness Undercut; tion during chromiumoi chromate film corheating in plating film Surface color rosron air(300 0., (micron) (micron) of product test 5 min.)

0.01 Metallic lustre. O No. 0.05 do O No. 0.1 do O No. 0.3 Bluish gray xTo yellow.

O means normal, 1 means that some peeling of the organic lacquer coatingas observed.

The undercut film corrosion test referred to in Table 1 was carried outby applying 30-40 mg./dm. of epoxy urea to the test piece, whereupon thethus coated test piece is baked at 200, C. for minutes. The coating wasthen cut or scratched with a needle in an X pattern formation and thethus cut portion-was extruded by 5 mm. with an Erichsen tester. The testpiece was then immersed in an aqueous solution containing 1.5% of citricacid and 1.5% of sodium chloride with the addition of a few drops offormalinper liter of solution. The immersion was effected for 5 days at80 F.

The various features of novelty which characterize the invention arepointed out with particularly in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which there is illustrated and described a preferredembodiment of the invention.

. 'BRIEF-DESCRIPTIO'N OF THE DRAWINGS Inthe drawingsi FIG. 1 shows on anenlarged scale and in diagrammatic manner a cross section of aninventive steel plate coated with a chromium layer and a chromate film,while FIG. 2 shows on an enlarged scale and diagrammatically a crosssection of a steel plate which has been treated in accordance with theprior art method as, for example, disclosed in US. Pat. 3,113,845.

DETAILED DESCRIPTION 'Tu'r ning now to FIG. 1, a relatively thin-gaugesteel *plate'is' indicated by reference numeral 1. A metallic chromiumlayer 2 is electrolytically plated onto the top and bottom surfaces ofthe plate, the subsequently electrolytically deposited chromate filmbeing indicated by reference numeral 3. The chromate film 3 is firmlybonded to' the'chromium layer 2. It will be noted that the pin holeareas 4 are also covered by the chromate film 3.

Referring now to FIG'I'Z which illustrates a prior art steel sheet, itwill be noted that the metallic chromium layer 2 is directly plated ontothe steel sheet 1. The dipping of the chromium plated steel sheet into achromic acid solution merely results in the formation of a chromatelayer 5 at the pin hole areas, but no chromate film is formed on themetallic chromium film 2.

The inventive chromium-chromate coated steel sheet is advantageouslyproduced according to the following method:

The surface of the steel plate is first degreased in any conventionalmanner. Electrolytic degreasing is thus feasible and indeed recommended.The plate is then washed and any rust is removed by acid pickling andadditional washing. The steel plate is then chromium electroplated in afirst stage in a chromium plating tank to obtain a chromium coating of0.00160.1 micron thick ness. The thus chromium plated sheet is thenwashed in hot water whereupon, in a second separate stage, the chromiumplated steel is subjected to an electrolytic treatment in which thechromium plated steel plate is the cathode. The electrolysis isperformed in an aqueous solution containing hexavalent chromium ionssuch as formed by chromic acid or dichromate or containing hexavalentchromium ions mixed with trivalent chromium ions. Reducing agents oragents accelerating and inducing chromate film formation on the metallicchromium coating may be added to the bath.

Concerning the composition of the electrolytic bath for effecting thechromate film deposition, it should be noted that the bath initially mayonly contain hexavalent chromium ions, or trivalent chromium ions may bepresent in admixture with the hexavalent chromium ions. However, itshould be appreciated that even if the bath initially containshexavalent chromium ions only, trivalent chromium ions will always beproduced due to the reducing action which takes place during theelectrolysis. The presence of weak reducing agents, of course, furtherpromotes the formation of trivalent chromium ions.

An important feature of the inventive procedure for forming the chromatefilm on the metallic chromium layer is that no sulfate ions or sulphuricacid radicals are permitted to be present in the bath which have atendency to deposit metallic chromium on the cathode, to wit, thepreviously chromium coated steel sheet.

Another important feature of the inventive procedure is that theelectrolytic deposition of the metallic chromium layer and thesubsequent electrolytic deposition of the chromate film are carried outin two distinct and separate stages. Thus, as will be apparent from theabove, the deposition of the chromium layer is carried out first andonly after the chromium layer has been deposited is the thus surfacetreated steel sheet subjected to the electrolytic deposition of thechromate film in the absence of sulfuric acid radicals or the likecatalysts which have a tendency to deposit metallic chromium.

As a general proposition, the conditions for depositing the two layers,to wit, the chromium layer and the chromate layer, may advantageouslybe, for example, as follows:

Conditions for formation of chromium layer Bath composition:

CrO 130-170 g./liter H2804: 0.2-0.4 g./litel Na SiF 3-7 g./liter Bathtemperature: 50i2 C. Current density: 20-80 amperes Treating time:1.25-5 seconds Time x density: 100.

Conditions for chromate film formation Bath composition:

CrO :5 g./liter Bath temperature: 55:5" C. Current density: 5 amperesTreating time: 0.5-2 seconds Comparative corrosion tests were carriedout with three different products, to wit:

A. a steel plate on which a chromate film was directly formed inelectrolytic manner;

B. a chromium plated steel plate which was dipped into chromic acid toform a chromate film; and

C. a steel plate manufactured in accordance with the present invention,to wit, having an electrolytically deposited chromium coating andsuperimposed thereon an electrolytically formed chromate film.

The tests were conducted before the products were coated with an organiccomposition. The results are shown in Table 2. The results of the saltspray corrosion test referred to in the Table are stated in terms ofhours up to rust formation while the results of the ferroxyl corrosiontests are given in terms of comparative values in which numeralrepresents that substantially no rust is formed while numeral 4represents that a few pit corrosion areas could be observed.

As is apparent from the results tabulated in Table 2, the inventivesteel plate exhibits a significantly improved corrosion resistance ascompared to that of the conventional surface treated steel plates, bothin the salt spray test and the ferroxyl test.

As previously stated, the primary industrial application of theinventive steel plate is as can stock and since can making material iscustomarily lacquered or varnished by an organic coating prior to use,tests were conducted for the purpose of determining the chemicalresistance of a can made from the inventive steel sheet and containing atop coat of organic material. Comparison tests with the prior artmaterials were also conducted. The results are tabulated in Table 3 andclearly indicate that the inventive steel plate has significantlyimproved characteristics as compared to those of the prior artmaterials.

TABLE 3 1. Lacquering with organic composition. Inside wall of can;phenol type varnish; baking at 200 0., minutes; amount 3540 up to pitrust formation). (4) 4% sulfuric acid immersion test hrs--. 3 hrs.

at room temp. (time up to pit rust formation). (5)- 0.4% caustic sodaboiling test (time up to put rust formation) (6) 10% CuClzimmersiontest, room First Second.

temp., 5 min. (7) Ferroxyl test, min- .do Do.

min 35 min.

filgorn 0.025 micron chromium plating and 0.005 mg./dm. chromate Theabove results are further discussed as follows: In a prior art steelplate which is directly subjected to electrolytic chromate treatmentwithout prior interposition of a chromium layer, the adherence betweenthe organic lacquer coating and the chromate film is satisfactory. Bycontrast, however, the adherence between the chromate film and the steelsurface is poor. Particularly, when immersed in citric acid andsulphuric acid, these acidic liquids penetrate through the organiclacquer film into and between the chromate film and the steel surface,thereby causing corrosion. This corrosion progresses until the organiclacquer coating peels off.

By contrast, in the inventive steel plate a metallic chromium layer ispositioned underneath the chromate film and this metallic chromium layerhas excellent corrosion resistance. The metallic chromium layertherefore acts as a strong protecting barrier or wall against corrosionof the steel surface proper and thus prevents corrosion even if theacidic liquid should penetrate through the lacquer coating and thechromate film. The bondor adherence between the steel surface an themetallic chromium layer is very strong and, furthermore, the adherenceor bond between the metallic chrominum layer and the superimposedchromate film is excellent. As a result, therefore, the metallicchromium film acts as a medium for enhancing the adherence between thechromate film and the steel surface, thus preventing penetration ofacidic liquor onto the steel surface, thereby improving the totalcorrosion resistance of the product.

Comparative corrosion tests were also conducted in respect to cans whichhad been produced from the inventive steel plates and which wereprovided with a top coat of organic composition. Comparison tests withprior art steel plates which had been first chromium plated andthereafter chromate treated by dipping or spraying were also conducted.The results are shown in the following Table 4.

TABLE 4 Sulfuric acid Citric acid test, in 5% Thickness Adhesion test,in 1% H2804 sol. of chromium of organic citric acid at room layerlacquer sol. boiling temp., 48 (micron) coating for 5 hrs. hrs.

Directly chromate 0 a: C 0

treated steel plate. Steel plate, 0.05 a: A B chromium 0.03 z B B platedand 0.01 z B B chromate 0.005 a: C B tireated by 0. 001 .2: D C

ping. Inventive steel 05 z A B 0.03 z A B plate having 0 01 z A B 0.05mg./dm. 0 6 z A B chromate film. 001 I A B Under 0.001 :2 B 0 Electrictin plate 1/ B 0 Exposure test, exposed Salt spray test, 48 hrs. Soytest, in soy on a rooftop for 10 at room temp. at 75 0.,

5 days days (3 rainy days incl.)

All of the test pieces were tested at a drawing rate of 5 mm. (Erichsenvalue) after they had been coated with an organic oil varnish of 10micron thickness. Thevalues in Table 4 have the following meaning: Arepresents no damage, B represents pit corrosion in the convex portionof the test piece; C represents peeling off over substantially theentire convex portion of the test piece; D represents corrosion over thewhole surface; x indicates that no peeling of the lacquer coating tookplace; y means slight peeling; and B indicates rust formation in thering portion of the Erichsen test.

It will be notedthat in a chromium plated steel plate which, wassubsequently chromatetreated by dipping, a lowering of the corrosionresistance was still observed if the, plating thickness was 0.005micron-0001 micron. By cont astrin-the inventive steelplate thecorrosion resistance is not affected down toa chromium layer thicknessof 0,001. For this reason, the lower limit for the thickness of themetallic chromium layer. should not be below 0.001 micronwhilethe upperlimit should not exceed 0.1 mieron. ,As previously stated, a lower limitof 0.0016 thickness .is, i howeyer,.preferred from a practical andprocesst'echnicalpointof' view. If the metallic chromium layer thicknessii eds J1- micron theprocessing of the can stock info cans isnegatively/affected. and inferior prod- Pe are o tai ed... I f,lt liernetallic chromium layerh'a's a thickness range of 0115,30.f0.-l,..rn'icroh, the steelfplate, as is clear from Table 2, exhibits'abetter corrosion resistance in nonlacquered statewthan. he, steel plate,corresponding to U.S.

ganic'compo'sition.

In the event that the chromium layer is thicker, cracks have a tendencyto appear during the manufacturing process' into pans because, as isknown, metallic chromium has a very significant hardnessrThis crackformation is counteracted in the inventive steel plate by the presenceof theelectrolyticallydeposited chromate film which cov- 'e'r's"theentire surface of the chromium layer including the pin hole portions.Since the'chromate film is relatively soft, any crack formation isavoided during bending and processing in gener'al.

The inventive steelplates can be easily processed and wtirked,'and,as"demonstrated above, have superior cor- "rosion' resistancecharacteristics. It" therefore provides a superior material,particularly for can stock, which is -produced at considerably lowercost than ordinary tin *platesJ'A wide variety "of cansand'can-likeproducts such as food cans, oil cans, detergentcans, crown caps, screw"caps, etc: may'besuccessfully manufactured from thesurface-treated:steelmaterial;

It2should. also be noted that -the inventive steel plates thaveremarkable characteristics .in that they are not susceptible to sulphurstain. In this connection it will be appreciated that sulphur-stainyisyadreaded phenomenon taking place in tin plates. Sincethe inventiveplates have better corrosion resistance non-lacquered state thanordinarychromium platedst eeljsheets, the inventive steel platesfean-beeconomically' produced even compared to the prior art chromium platedsteels, as'a thinner layer ofchromiumis in most cases sufficient. Inaddition, of abana-me inventive plates have improved coating adher-'ence '-and -corrosion -resistance, "particularly if compared with priorart .chromate treated steel.

-According to afurtherfeature of the .invention, the in- "ventiveiprocedure is.particularly suitable for surface treating=matteiinishedsteel sheets-of the nature disclosed in patent applicationSeriNo. 610,025, filed Jan. 18,

1967. Thus, the inventive'procedure is applicable for ob- -;tainingscratch-resistant. white silver'chromiumplated steel plates of thenature disclosed in said application wherein the steel plate has beenmatte finished. In accordance with this invention, the chromate film isthen electrolytically superimposed on the chromium film. The surfaceroughness of the matte finished initial steel plate may be 10 EXAMPLE 1.Pretreatment A cold rolled steel sheet of 0.27 mm. thickness issubjected to electrolytic degreasing in a 5% caustic soda solution at C.and at a current density of 20 A./dm.'. The electrolytic degreasingtreatment is carriedout for 2 seconds, whereupon the sheet is washed.The sheet is then subjected to acid pickling in a 10% sulphuric acidsolution for 2 seconds whereupon further washing is effected.

2. Chromium plating The thus cleaned steel sheet is then employed as thecathode in an electrolytic circuit, the electrolytic bath containing 200g./l. of chromic' acid and 20 g./l. of sulphuric acid. Theelectroplating is carried out at 50 C. and with a current density of 40A./dm. for 1 second.

3. Chromate layer formation After washing, the chromium plated sheet isemployed as the cathode in an electrolytic circuit, the electrolyticbath being a chromate treating liquor containing 50 g./l. of chromicacid and 2 g./l. of sodium silicofluoride. The temperature of the bathwas 40 C. and the chromate deposition was effected under a currentdensity of 5 A./ dm. for 2 seconds. The thus electroplated sheet wasthen washed, dried and oiled.

4. Organic coating The inventive steel sheet may be lacquered orvarnished with an organic coating composition which may, for example, bephenol based, oil based, alkyd-based, vinyl resin based or epon or epoxybased. Varnishes and lacquers as disclosed, for example, in U.S. Pat.3,113,845 may be used. It will be appreciated that generally anysuitable coating composition used as top layer in cans may be employedfor the inventive purposes, the various coating compositions being usedeither singly or in combination.

Synthetic resin coatings of the vinyl type, phenol type and polyethylenetype may be used as lacquers with or without adhesives.

Recently, a one-step procedure has been proposed according to which asteel sheet is cathodically plated in an aqueous bath of chromicanhydride containing trivalent chromium ions and a sulphuric acidradical in specific amount. It is alleged that this procedure results inthe formation of an intermediate metallic chromium layer and anon-metallic chromate top layer. This procedure has been disclosed inU.S. Pat. 3,296,100. Since from a practical point of view it is at thepresent state of analytical knowledge not possible to distinguishbetween a metallic chromium layer and a superimposed chromate layer, ifthe layers are extremely thin, the alleged result cannot be confirmed orvertified and is, in fact, questionable. In any event, the presentprocedure is quite different from that disclosed in U.S. Pat. 3,296,100.

As the composition of the treating liquids and "the process conditionsare different and in accordance with the present procedure, the chromiumlayer and the chromate film are applied-in two separate steps whileaccording to the process referred to both layers are allegedly formed inthe same step, different results are obtained. Moreover, according toU.S. Pat. 3,296,100, the treating solution contains a sulphuric acidradical while according to the present procedure the chromate treatmentmust be effected in the absence of such radicals in order to avoid thedeposition of metallic chromium.

As set forth, it is presently not possible to identify the chemicalcomposition of the layers allegedly formed according to U.S. Pat.3,296,100 since these layers are extremely thin and amorphous. However,since the bath compositions are different, it must be reasonably assumedthat the chemical composition of the layers in the U.S. patent referredto and in the present steel sheet are dis- TABLE Steel late Steel plateas sur ace surface treated treated by according to present U.S. patentImmersion solution invention 3,296,100

Ion exchange water 0. 37 3% NazCOa 0.71 3% NaCl 0.15 50% ethyl alcoholTr. 0.60

Tr. represents 0.01 p.p.m.

The significant ditferences in the amount of dissolved chromium asappearing in Table 5 indicate that there is an essential difference inthe chemical composition of the top layers.

In this connection it should again be emphasized that the thickness ofthe chromate film as disclosed herein is critical and that the thicknessof this film according to the present procedure is considerably greaterthan that of the U.S. patent. As is clear from this figure of the U.S.patent, when both a metallic chromium layer and a chromate top layer aresimultaneously formed in a one-step procedure, as alleged in the U.S.patent, the thickness of the metallic chromium layer must have a certainratio in respect to that of the chromate film. This ratio is 2 mg./dm.(0.0275 micron to 1.8 mg./dm. Although this ratio may vary slightly, itis not possible to produce a surface treated steel plate having athickness ratio which is essentially different from the indicated ratio.Based on this ratio, if the thickness of the top layer is less than 0.1mg./dm. the thickness of the metallic chromium layer is always less than0.0015 micron.

Since the process of the present invention is a two-step process, it isthus feasible to apply a thicker metallic chromium layer (0.0016-0.1micron) and a thinner top layer (0.1 mg./dm. Cr). Experiments haveindicated that in the present procedure the best results are obtained ifthe electrolytic chromate deposition is carried out for 0.5-2 secondsonly. Longer periods than 2 seconds should be avoided.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the inventiveprinciples, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:

1. Can stock consisting essentially of a thin-gauge steel sheet, saidsteel sheet having directly electro-plated thereon a metallic chromiumlayer of a thickness not exceeding about 0.1 micron, and a chromate filmelectrolytically deposited on said chromium layer, said chromate filmhaving a thickness of not more than about 0.1 mg./dm. said can stockhaving the following characteristics:

(at) its plated surface exhibits metallic lustre, as distinguished froma bluish or'gray tint;'

(b) its plated surface does not discolorupon'heating in air attemperatures up to 300 C.; (c) its plated surface does not formrust'spots for'at least 20 hours if subjected to'thesalt spray corrosiontest pursuant to JIS Z-237l; Y T

(d) the plating does not crack if the can stock is manufactured intosquare'cans,

(e) organic coatings adhere to it' without peeling;

(f) trace amounts of chromium of the order of'- 0.0l

p.p.m. dissolve when'3 dmfiof the can'stock'is immersed in an aqueoussolution containing"3-%' -of Na CO 3% of NaCl and 50% of ethylalco'holfor 24 hours at a temperatureof 50 C.; and (g) its platedsurface exhibits lacquerability and paintability after boiling for 3hours in watercontaining 1% of citric acid upon extrusion atan'Erichsen'value of5mm.

2. Can stock as claimed in claim 1 wherein the thickness of the chromiumlayer is at least about 0.0016 micron.

3. Can stock as claimed in claim 1, wherein said chromate film isdeposited on said chromium layer from'a chromium containing treatingliquid devoid of a sulphuric acid radical catalyst inducing metallicchromium deposition.

4. Can stock as claimed in claim 1, wherein said chromate filmessentially consists of trivalent chromium oxide and its hydrates.

5. Can stock as claimed in claim 1, wherein said chromate filmessentially consists of trivalent chromium oxide and its hydrates and ofhexavalent chromium oxide and its hydrates.

6. Can stock as claimed in claim 1, wherein an organic coating issuperimposed on said chromate film.

7. Can stock as claimed in claim 1, wherein said chromium layer isplated onto a steelplate whichhasbeen matte finished.

8. Can stock as claimed in claim 1, wherein said matte finished steelplate has a surface roughness of about between 0.8-3 microns in terms ofHr.m.s. v

9. Can stock as claimed in claim 1, wherein said chromate film isessentially free of metallic chromium.

10. Can stock as in claim 1, wherein said'chromate film covers andadheres to substantially. the entire.- chromium layer. Y r

References Cited 1 UNITED STATES PATENTS" 2,746,915 5/1956v Giesker204+56 2,998,361 8/1961 Kitamura '204--56 3,113,845 10/1963 ,Uchida204--.41 3,257,295 6/1966 Yonezaki .204-56 3,288,691 11/1966 Yonezaki,2041-56 3,245,885 4/1966 Asano 2 204-756 3,296,100 1/ 1967 :Yonezaki204--56 3,296,106 1/1967 Smith ...-....;...a 204-56 3,316,160 4/1967Uchida 204-+41 3,479,260 11/1969 Rauch 204-166 3,519,542 7/ 1970Inventofls) v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONAugust 6, 1974 Patent No. 3 827,866 Dated Hiromn Uchida and OsamuYanabu,

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as "shown below:

In. the'I-Ieading of the Patent, insert:

--C1aims Priority applications Japan March 26, 1966, sho 41-18522November ll 1966 sho 4l-74l47--.

Signed end sea led this 19th day of November 1974.

I 3 (SEAL) I Attest:

C. MARSHALL DANN McCOY M. GIBSON JR.

Commissioner of Patents Attesting Officer 1 FORM PO-IOSO (10-69)USCOMM-DC OOSIB-PGQ w u.s. GOVERNMENT PRINTING onlc: nu o-al-su,

